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Hydrogen cyanide (HCN) aqueous solutions can remove copper contaminants from Si surfaces more effectively than hydrochloric acid/hydrogen peroxide mixture (HPM) and sulfuric acid/hydrogen peroxide mixture (SPM). When pH of the HCN solutions is adjusted at 9, Si surface morphology is not changed, while when pH exceeds 10, the Si surfaces are considerably roughed. AFM measurements show that Cu contaminants are present in the form of particles on the bare Si surfaces. XPS measurements show that the particles consist of metallic Cu. The Cu particle height decreases almost linearly with the cleaning time, and the Cu surface concentration decreases exponentially with it. It is concluded that Cu particles gradually dissolve into the HCN aqueous solutions by the direct reaction with cyanide ions at the surface of the Cu particles.

A Si cleaning method has been developed by use of potassium cyanide (KCN) dissolved in methanol. When silicon dioxide (SiO2)/Si(1 0 0) specimens with 1014 atom/cm2 order copper (Cu) contaminants are immersed in 0.1 M KCN solutions of methanol at 25 °C, the Cu concentration is reduced to below the detection limit of total X-ray fluorescence spectrometer of ∼3 × 109 atoms/cm2. X-ray photoelectron spectra show that the thickness of the SiO2 layers is unchanged after cleaning with the KCN solutions. 1014 cm−2 order Cu contaminants on the Si surface can also be removed below ∼3 × 109 atoms/cm2, without causing contamination by potassium ions. UV spectra show that Cu-cyano complex ions are formed in the KCN solutions after the cleaning. The main Cu species in the KCN solutions is Cu ( CN ) 4 3 - ions with the concentration of [ Cu ( CN ) 4 3 - ]:[Cu+] = 1:1.6 × 1023. Even when the KCN solutions are contaminated with 64 ppm Cu2+ ions in the solutions, which form Cu ( CN ) 4 3 - ions, the cleaning ability does not decrease, showing that Cu ( CN ) 4 3 - ions are not re-adsorbed. The KCN solutions can also passivate defect states such as Si/SiO2 interface states, leading to the improvement of characteristics of Si devices.

Absract Hydrogen cyanide (HCN) aqueous solutions are found to possess a great ability of removal of copper contaminants on Si as well as a Si defect passivation effect. Copper (Cu) contaminants of 10 13 cm −2 order concentrations can be reduced below 3×10 9 cm −2 taking only 10 s even at room temperature when the pH of the HCN aqueous solution is set at 10. The cleaning ability of HCN solutions increases with pH, indicating that cyanide ions are the reaction species with Cu contaminants. Even when HCN solutions are contaminated with 64 ppm Cu 2+ ions, the cleaning ability is unchanged, indicating avoidance of re-adsorption. Ultraviolet absorption spectra show that after the cleaning, Cu–cyanide complexes are formed in the HCN solutions. The main Cu species in the HCN solutions are Cu ( CN ) 3 2 − , and due to its stability, re-adsorption does not occur. When SiO 2 /Si structure is immersed in the HCN aqueous solutions for 24 h, the SiO 2 thickness is unchanged at all, showing that etching does not occur during the cleaning process. The energy conversion efficiency of pn-junction Si solar cells with a spherical shape is increased to 1.07–1.48 times by immersion in the HCN aqueous solutions, and the improvement is attributed to passivation of defect states in Si by cyanide ions.

The cleaning method using cyanide solutions has been developed to remove heavy metals such as copper (Cu) and nickel (Ni) from Si surfaces. Immersion of Si wafers with both Cu and Ni contaminants in potassium cyanide (KCN) solutions of methanol at room temperature decreases these surface concentrations below the detection limit of total reflection X-ray fluorescence spectroscopy of ∼3×10 9 atoms/cm 2 . UV spectra of the KCN solutions after cleaning of the Cu-contaminated Si surface show that stable copper-cyanide complexes are formed in the solution, leading to the prevention of the re-adsorption of copper in the solutions. From the complex stability constants, it is concluded that the Cu(CN) 4 3− is the most dominant species in the KCN solutions.

Hydrogen cyanide (HCN) aqueous solutions can remove copper (Cu) contaminants from SiO 2 surfaces completely even when the cleaning is performed using dilute (e.g., 0.027 wt %) HCN aqueous solutions at 25°C. When pH of the HCN solutions is set at 10, Cu contaminants with a concentration of ∼2 X 10 12 atoms/cm 2 can be removed below ∼3 × 10 9 atoms/cm 2 taking only 10 s. The concentrations of Cu + and [Cu(CN) 2 ] - ions in the HCN solutions drastically decrease with pH, while those of [Cu(CN) 3 ] 2- and [Cu(CN) 4 ] 3- ions increase. The dissociation probability of HCN exponentially increases with pH.Therefore, the improvement of the cleaning ability with pH is attributable to (i) an increase in the concentration of CN - ions, and (ii) the prevention of readsorption due to the formation of stable Cu-cyano complex ions. Measurements of surface Cu concentrations on the SiO 2 surfaces contaminated by the immersion in the CuCl 2 -containing HCN solutions clarify the mechanism of Cu removal. The rate-determining step is a reaction between SiO-CuCN on the surface and CN - ions in the solution to form [Cu(CN) 2 ] - complex ions. The equilibrium constant of this reaction is found to be 4.1 x 10 5 M. This large value shows that desorption of [Cu(CN) 2 ] - proceeds much more easily than adsorption.